1. Field of the Invention
The present invention relates to a nuclear fuel rod of uranium dioxide pellets containing gadolinium oxide or gadolinia, filled up in a cladding tube.
2. Description of the Background Art
In a nuclear electric power generation, by burning uranium dioxide to generate heat, water is heated by the heat to generate steam of high temperature and a steam turbine is driven by the steam to produce electric power. In order to control a nuclear reaction, control rods are used, or gadolinium oxide or gadolinia for absorbing neutrons is admixed in uranium dioxide pellets to control the reaction, in particular, in its beginning step. The gadolinia is known as a flammable poison, and, once the gadolinia once absorbs the neutrons, its neutron-absorbing ability is gone. Thus, the gadolinia is essential for controlling the reaction speed in the beginning step. Usually, several number of nuclear fuel rods which are entirely or partially filled with pellets of uranium dioxide admixed with several percent of gadolinia, are used for each fuel assembly composed of 63 fuel rods.
In such a nuclear fuel rod, a cladding tube is made of a metallic material, e.g., a zirconium alloy such as Zircaloy-2 or the like, and contains uranium dioxide pellets, and upper and lower plugs stop the upper and lower openings of the cladding tube. In the cladding tube, a stopper plate for stopping the pellets and a spring for biasing the stopper plate onto the pellets are arranged. In some uranium dioxide pellets, usually, several percent of gadolinia is contained. A getter tube is also arranged between the upper plug and the uppermost pellet in the cladding tube, and a moisture getter is sealed within the getter tube. In this case, the structural members such as the spring, the stopper plate and the getter tube are made of a metallic material such as AISI 302 stainless steel or the like.
In general, structural members mounted in the nuclear reactor are carefully and severely inspected. The nuclear fuel rod is, of course, inspected after the fabrication, for instance, by passing it through a strong magnetic field of scores of thousands of gauss in a magnetic and passive scanner (MAPS) to inspect how many percent of gadolinia is contained in the uranium dioxide pellets. In such an inspection, since the structural members, i.e., the spring, the stopper plate and the getter tube are made of the stainless steel, as described above, they are usually magnetized to 100 to 200 gauss when passing through the strong magnetic field of scores of thousands of gauss, and their magnetization can be observed from the outside of the cladding tube. The magnetization of the spring, the stopper plate and the getter tube in the nuclear fuel rod brings about the following problems.
That is, in a long period of operation of the nuclear reactor, suspended materials such as hematite, magnetite and the like are produced in water, and the suspended materials contact with and attach onto the outer surface of the nuclear fuel rod as clads to cover the surface of the nuclear fuel rod. This covering of the nuclear fuel rod prevents the heat conduction or transference of the nuclear fuel rod to increase the temperature of the cladding tube of the nuclear fuel rod and thus to accelerate corrosion of the cladding tube. The magnetite of the suspended materials is an iron oxide having magnetic property, and hence, when the parts such as the spring, the stopper plate and the getter tube in the nuclear fuel rod are magnetized, the magnetite is much more liable to attach to the surface of the nuclear fuel rod, resulting in the remarkable acceleration of the corrosion of the cladding tube.